Author Code broken?

I’ve created a filter for the author info to pull a guest author from custom fields.

function fb_guest_author($author,$post_id){
	
	global $post;
	//global $author;
	   
	   $post = get_post($post_id);
	   $author = get_post_meta($post->ID, 'guest_author', true);
	   return $author;
}

add_filter( 'afbia_author', 'fb_guest_author', 10,2);

The function works but the resulting Instant Article code is garbled e.g

<address>
<a gordon="" gregory data-wpel-link="internal">
</a>
</address>

I’ve had a look at the code in instant_article.php around line 140 where the <address> code is but can’t work out why it’s not parsing correctly. The only way I can get it to work is replace the whole code block with

<address>
<a><?php echo esc_html($author); ?></a></address>

Any ideas and to what would cause this?